Fix potential overflow in decrement when computing GC_markers_m1

Fix potential overflow in decrement when computing GC_markers_m1

Also, call WARN if a non-positive value is specified in GC_MARKERS.

* pthread_support.c [PARALLEL_MARK] (GC_thr_init): Replace markers_m1
local variable with markers one; keep real number of markers in
"markers" variable (not a decremented one); treat invalid (i.e.
non-positive) markers value (obtained from GC_MARKERS environment
variable) the same way as too big ones (i.e. set to maximum number of
markers in this case); adjust WARN message accordingly; report invalid
or too big markers value in WARN.
* win32_threads.c [PARALLEL_MARK] (GC_thr_init): Likewise.
* pthread_support.c [PARALLEL_MARK] (GC_thr_init): Adjust code
indentation.